Photodiode and photodiode array with improved performance characteristics

ABSTRACT

The present invention is a photodiode and/or photodiode array, having a p+ diffused area that is smaller than the area of a mounted scintillator crystal, designed and manufactured with improved device characteristics, and more particularly, has relatively low dark current, low capacitance and improved signal-to-noise ratio characteristics. More specifically, the present invention is a photodiode and/or photodiode array that includes a metal shield for reflecting light back into a scintillator crystal, thus allowing for a relatively small p+ diffused area.

CROSS-REFERENCE

The present application is a continuation of U.S. patent applicationSer. No. 12/199,558, filed on Aug. 27, 2008, and now U.S. Pat. No.7,709,921.

FIELD OF THE INVENTION

The present invention relates generally to the field of radiationdetectors, and in particular, to a photodiode and/or photodiode arrayhaving improved device performance characteristics. Specifically, thepresent invention relates to a photodiode and/or photodiode array,having a p+ diffused area that is both relatively small and smaller thanthe area of a mounted scintillator crystal, designed and manufacturedwith improved device characteristics, and more particularly, relativelylow dark current, low capacitance and improved signal-to-noise ratiocharacteristics. More specifically, the present invention relates to aphotodiode and/or photodiode array that includes a metal shield forreflecting light back into a scintillator crystal, thus allowing for arelatively small p+ diffused area.

BACKGROUND OF THE INVENTION

Active solid-state semiconductor devices, and in particular, siliconphotodiodes, are among the most popular photodetectors having asufficiently high performance over a large wavelength range and asufficient ease of use. Silicon photodiodes are sensitive to light inthe wide spectral range, extending from deep ultraviolet through visibleto near infrared, which is approximately 200 nm to 1100 nm. Siliconphotodiodes, by using their ability to detect the presence or absence ofminute light intensities, facilitate the precise measurement of theseminute light intensities upon appropriate calibration. For example,appropriately calibrated silicon photodiodes detect and measure lightintensities varying over a wide range, from very minute lightintensities of below 10⁻¹³ watts/cm² to high intensities above 10⁻³watts/cm².

Silicon photodiodes can be employed in an assortment of applicationsincluding, but not limited to, spectroscopy, distance and speedmeasurement, laser ranging, laser guided missiles, laser alignment andcontrol systems, optical free air communication, optical radar,radiation detection, optical position encoding, film processing, flamemonitoring, scintillator read out, environmental applications such asspectral monitoring of earth ozone layer and pollution monitoring, lowlight-level imaging, such as night photography, nuclear medical imaging,photon medical imaging, and multi-slice computer tomography (CT)imaging, security screening and threat detection, thin photochipapplications, and a wide range of computing applications.

Typically, photodiode arrays employ a scintillator material forabsorbing high energy (ionizing) electromagnetic or charged particleradiation, which, in response, fluoresces photons at a characteristicwavelength. Scintillators are defined by their light output (number ofemitted photons per unit absorbed energy) short fluorescence decaytimes, and optical transparency at wavelengths of their own specificemission energy. The lower the decay time of a scintillator, that is,the shorter the duration of its flashes of fluorescence are, the lessso-called “dead time” the detector will have and the more ionizingevents per unit of time it will be able to detect. Scintillators areused to detect electromagnetic waves or particles in many security anddetection systems, including CT, X-ray, and gamma ray. There, ascintillator converts the energy to light of a wavelength which can bedetected by photomultiplier tubes (PMTs) or P-N junction photodiodes.

Photodiodes are typically characterized by certain parameters, such as,among others, electrical characteristics, optical characteristics,current characteristics, voltage characteristics, and noise. Electricalcharacteristics predominantly comprise shunt resistance, seriesresistance, junction capacitance, rise or fall time and/or frequencyresponse. Optical characteristics comprise responsivity, quantumefficiency, non-uniformity, and/or non-linearity. Photodiode noise maycomprise, among others, thermal noise, quantum, photon or shot noise,and/or flicker noise.

In an effort to increase the signal to noise ratio and enhance thecontrast of the signal, it is desirable to increase the light-inducedcurrent of photodiodes. Thus, photodiode sensitivity is enhanced whilethe overall quality of the photodiode is improved. Photodiodesensitivity is crucial in low-level light applications and is typicallyquantified by a parameter referred to as noise equivalent power (NEP),which is defined as the optical power that produces a signal-to-noiseratio of one at the detector output. NEP is usually specified at a givenwavelength over a frequency bandwidth.

Photodiodes absorb photons or charged particles, facilitating detectionof incident light or optical power and generating current proportionalto the incident light, thus converting the incident light to electricalpower. Light-induced current of the photodiode corresponds to the signalwhile “dark” or “leakage” current represents noise. “Dark” current isthat current that is not induced by light, or that is present in theabsence of light. Photodiodes process signals by using the magnitude ofthe signal-to-noise ratio.

Leakage current is a major source of signal offset and noise in currentphotodiode array applications. Leakage current flows through thephotodiode when it is in a “dark” state, or in the absence of light at agiven reverse bias voltage applied across the junction. Leakage currentis specified at a particular value of reverse applied voltage. Leakagecurrent is temperature dependent; thus, an increase in temperature andreverse bias results in an increase in leakage or dark current. Ageneral rule is that the dark current will approximately double forevery 10° C. increase in ambient temperature. It should be noted,however, that specific diode types can vary considerably from thisrelationship. For example, it is possible that leakage or dark currentwill approximately double for every 6° C. increase in temperature.

In certain applications, it is desirable to produce optical detectorshaving small lateral dimensions and spaced closely together. For examplein certain medical applications, it is desirable to increase the opticalresolution of a detector array in order to permit for improved imagescans, such as computed tomography (CT) scans. However, at conventionaldoping levels utilized for diode arrays of this type, the diffusionlength of minority carriers generated by photon interaction in thesemiconductor is in the range of at least many tens of microns, and suchminority carriers have the potential to affect signals at diodes awayfrom the region at which the minority carriers were generated.

Thus, an additional disadvantage with conventional photodiode arrays isthe amount and extent of crosstalk that occurs between adjacent detectorstructures, primarily as a result of minority carrier leakage currentbetween diodes. The problem of crosstalk between diodes becomes evenmore acute as the size of the detector arrays, the size of individualdetectors, the spatial resolution, and spacing of the diodes is reduced.

Various approaches have been used to minimize such crosstalk including,but not limited to, providing inactive photodiodes to balance theleakage current, as described in U.S. Pat. Nos. 4,904,861 and 4,998,013to Epstein et al., the utilization of suction diodes for the removal ofthe slow diffusion currents to reduce the settling time of detectors toacceptable levels, as described in U.S. Pat. No. 5,408,122, andproviding a gradient in doping density in the epitaxial layer, asdescribed in U.S. Pat. No. 5,430,321 to Effelsberg.

In addition to leakage current and effects of crosstalk, noise is oftena limiting factor for the performance of any device or system. In almostevery area of measurement, the limit to the detectability of signals isset by noise, or unwanted signals that obscure the desired signal. Asdescribed above, the NEP is used to quantify detector noise. Noiseissues generally have an important effect on device or system cost.Conventional photodiodes are particularly sensitive to noise issues.Like other types of light sensors, the lower limits of light detectionfor photodiodes are determined by the noise characteristics of thedevice.

As described above, the typical noise components in photodiodes includethermal noise; quantum or shot noise; and flicker noise. These noisecomponents collectively contribute to the total noise in the photodiode.Thermal noise, or Johnson noise, is inversely related to the value ofthe shunt resistance of photodiode and tends to be the dominant noisecomponent when the diode is operated under zero applied reverse biasconditions. Shot noise is dependent upon the leakage or dark current ofphotodiode and is generated by random fluctuations of current flowingthrough the device, which may be either dark current or photocurrent.Shot noise tends to dominate when the photodiode is used inphotoconductive mode where an external reverse bias is applied acrossthe device. As an example, detector noise generated by a planar diffusedphotodiode operating in the reverse bias mode is a combination of bothshot noise and thermal noise. Flicker noise, unlike thermal or shotnoise, bears an inverse relationship to spectral density. Flicker noisemay dominate when the bandwidth of interest contains frequencies lessthan 1 kHz.

Secondary issues also contribute to dark noise and other noise sourcesthat impact photodiode sensitivity. These include primarilydetermination and/or selection of apt active area specifications(geometry and dimensions), response speed, quantum efficiency at thewavelength of interest, response linearity, and spatial uniformity ofresponse, among others.

Further, when a photodiode is used in conjunction with an electronicamplifier, the capacitance of the photodiode can be a dominatingcontributing characteristic in the overall noise. It is thereforedesirable to design and fabricate photodiodes that have low overallcapacitance.

In CT applications, such as those employed for baggage screening, it isdesirable to have high density photodiode arrays with low dark current,low capacitance, high signal-to-noise ratio, high speed and lowcrosstalk.

As mentioned above, however, there are numerous problems withconventional photodiodes that attempt to achieve these competing andoften conflicting characteristics. Referring now to FIGS. 1 a and 1 b,top and cross-sectional views, respectively, of a conventionalphotodiode are shown. The photodiode shown in FIGS. 1 a and 1 b istypical in that it is fabricated such that the p+ diffused area has thesame dimensions as the scintillator crystal. For example, and asdescribed in greater detail below, if the scintillator crystal has alength of 2 mm and a width of 2 mm, then the p+ diffused area is also 2mm×2 mm. This is a typical design characteristic for photodiodes used inx-ray/scintillator applications.

Referring to FIG. 1 a, conventional photodiode 100 comprises substratematerial 102, which is typically a bulk silicon substrate lightly dopedwith a suitable impurity of a selected conductivity type, such as p-typeor n-type. In order to meet desired capacitance performancespecification, the device is fabricated on a n-type silicon substratewafer that is of high resistivity, typically on the order of 4500-9000ohm cm. The photodiode comprises a shallow p+ diffused region 110, deepp+ diffused region 112, and anode metal pads 114 a for forming at leastone contact and 114 b for forming at least one wire bond. It should benoted that, in a typical photodiode chip, distance 129 between the p+diffused area 110 and the edge 130 of the photodiode 100 ranges from 0.1mm to 0.125 mm.

Referring to FIG. 1 b, conventional photodiode 100 comprises a shallowp+ diffused region 110, which has a top surface area of 2 mm×2 mm.Conventional photodiode 100 further comprises a scintillator crystal120, which is of the same dimensions as the p+ active area/diffusedregion 110 as described above, or 2 mm×2 mm, mounted on the photodiodeusing an epoxy. Conventional photodiode 100 also comprises deep p+diffused region 112; anode metal pads 114 a for forming at least onecontact and 114 b for creating at least one wire bond; and a layer 115comprised of silicon oxide/silicon nitride that acts as junctionpassivation and antireflection layer.

The conventional photodiode shown in FIGS. 1 a and 1 b are, however,fabricated on an expensive, high resistivity silicon substrate material102 to accommodate for a large p+ diffused area while still retaininglow overall capacitance and thus, improved performance. Further, a largep+ diffused area has high dark current since the amount of dark currentis proportional to the size of the p+ diffused area, and thus, a lowsignal to noise ratio. Still further, because the p+ diffused area islarge, there is little distance between the p+ diffused area and theedge of the chip, there is an increase in sawing damage and thus,overall yield may be relatively low.

Thus, conventional photodiodes suffer from low signal-to-noise ratiosand sub-optimal characteristics. Conventional photodiodes are also knownto have high cost of manufacturing and low yield.

Therefore, what is needed is a photodiode and photodiode array that canbe manufactured on a low resistivity substrate wafer.

What is also needed is a photodiode and photodiode array that can bemanufactured with a relatively small p+ diffused active area to obtainlow capacitance while still retaining high signal current.

In addition, there is need in the art for photodiodes that have improvedoverall performance characteristics, allow for high yield and can beinexpensively manufactured.

SUMMARY OF THE INVENTION

In one embodiment, the present invention is a photodiode comprising atleast a front side and a back side, fabricated from a low resistivitysubstrate, having a resistivity in the range of 1000 ohm cm to 2000 ohmcm, further comprising a shallow p+ diffused region on the front side,wherein the shallow p+ region is smaller than a scintillator crystalmounted on the front side of the photodiode; at least one back sidecathode contact and at least one front side anode contact, comprised ofmetal; and a reflective metal shield on at least a portion of said frontside, wherein said metal shield reflects incident light back into saidscintillator crystal.

In one embodiment, the present invention is a photodiode arraycomprising a low resistivity substrate having at least a front side anda back side, wherein said low resistivity is in the range of 1000 ohm cmto 2000 ohm cm; a plurality of diode elements integrally formed in thesubstrate forming said array, wherein each diode element has a shallowp+ diffused region on said front side, wherein said shallow p+ region issmaller than a scintillator crystal mounted on the front side of thephotodiode; at least one back side cathode contact and at least onefront side anode contact, comprised of metal; and a reflective metalshield on at least a portion of said front side, wherein said metalshield reflects incident light back into the scintillator crystal.

In one embodiment, the scintillator crystal mounted on the front side ofthe photodiode has a top surface area of 2 mm×2 mm and the shallow p+diffused region is on the order of 1 mm or less in length and 1 mm orless in width.

In one embodiment, the reflective metal shield comprises aluminum.

In one embodiment, a distance between the shallow p+ diffused region andan edge of the photodiode ranges from 0.3 mm to 0.5 mm.

The photodiode of the present invention further comprises ananti-reflective layer which, in one embodiment, has a thickness of 900Å. In another embodiment, the anti-reflective layer is formed from SiO₂having a thickness of 150 Å and Si₃N₄ having a thickness of 400 Å, for atotal layer thickness of 550 Å.

In one embodiment, the present invention describes a method offabricating a photodiode on a low resistivity substrate wafer, on theorder of 1000 ohm cm to 2000 ohm cm, having a front side and a backside, said method comprising the steps of: providing an oxide layer onboth the front side and the back side, having a thickness of 10,000 Å;implementing an etching process on both the front side and back side todefine a plurality of regions on the front side and back side; fillingthe plurality of regions with n+ dopant; depositing an oxide layer onboth the front side and the back side of the wafer; creating a deep p+diffused region on the front side; fabricating an active area on saidfront side, wherein said active area is smaller than a scintillatorcrystal mounted on the front side of the photodiode; forming ananti-reflective layer on the front side of said device; forming ashallow p+ active area region; forming at least one contact window; andperforming a metal deposition process to etch metal on both the frontside and back side of the device wafer, wherein said metal depositionprocess creates connections and wherein said metal deposition processforms a reflective metal shield on the front side, that reflectsincident light back into the scintillator crystal mounted on thephotodiode.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated, as they become better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings wherein:

FIG. 1 a is a top view of a conventional, prior art photodiode;

FIG. 1 b is a cross-sectional view of a conventional, prior artphotodiode with a scintillator crystal mounted on the front side of thephotodiode;

FIG. 2 a is a top view of one embodiment of a photodiode of the presentinvention;

FIG. 2 b is a cross-sectional view through the center of one embodimentof a photodiode of the present invention, with a scintillator crystalmounted on the front surface of the photodiode;

FIG. 3 is a cross-sectional view of one embodiment of photodiode of thepresent invention, illustrating light reflected from the metal shieldundergoing multiple reflections from within the mounted scintillatorcrystal;

FIG. 4 a is a cross-sectional of a low resistivity substrate wafer aftera mask oxidation step, in one embodiment of the manufacture of thephotodiode of the present invention;

FIG. 4 b is a cross-sectional view of the wafer shown in FIG. 4 a, aftera n+ masking, phosphorus diffusion, and front and back side oxidationstep, in one embodiment of the manufacture of the photodiode of thepresent invention;

FIG. 4 c is a cross-sectional view of the wafer shown in FIG. 4 b, aftera front side p+ masking and deep boron diffusion step, in one embodimentof the manufacture of the photodiode of the present invention;

FIG. 4 d is a cross-sectional view of the wafer shown in FIG. 4 c, afteractive area masking, anti-reflective layer growth, and boron implant andannealing steps, in one embodiment of the manufacture of the photodiodeof the present invention;

FIG. 4 e is a cross-sectional view of the wafer shown in FIG. 4 d, aftera contact window masking step, in one embodiment of the manufacture ofthe photodiode of the present invention; and

FIG. 4 f is a cross-sectional view of the wafer shown in FIG. 4 e, afterat least one metallization step, in one embodiment of the manufacture ofthe photodiode of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed towards a photodiode and/or photodiodearray having improved device characteristics, such as low capacitance,low dark current, improved signal-to-noise ratio, low resistivitystarting material, and lower fabrication and manufacturing costs.

In one embodiment, the present invention is directed towards aphotodiode characterized by a relatively small p+ diffused area. In oneembodiment, the present invention is directed towards a photodiodecharacterized by a p+ diffused area that is smaller than the area of ascintillator crystal that is mounted on the top surface of thephotodiode.

In one embodiment, the present invention is directed towards aphotodiode having a relatively small p+ diffused area, morespecifically, on the order of 1 mm×1 mm or less, whereby the photodiodehas reduced dark current and an improved signal-to-noise ratio. In oneembodiment, the p+ diffused area of the photodiode is 1 mm×1 mm.

In one embodiment, the present invention is directed towards aphotodiode and/or photodiode array in which at least a portion of thefront or top surface of the photodiode or photodiode array is coveredwith a metal shield, employed as a reflective surface to reflectincident light back into a scintillator crystal mounted on thephotodiode. In one embodiment, the metal shield is fabricated fromaluminum.

In one embodiment, the present invention is directed towards aphotodiode and/or photodiode array having a relatively small p+ diffusedactive area, in which at least a portion of the front or top surface ofthe photodiode or photodiode array is covered with a metal shield,employed as a reflective surface to reflect incident light back into ascintillator crystal mounted on the front side of the photodiode. In oneembodiment, the relatively small p+ diffused area is smaller than thearea of the scintillator crystal mounted on the front side of thephotodiode.

In one embodiment, the reflective surface increases the performance ofthe photodiode and compensates for the smaller p+ diffused active areain terms of performance characteristics. Thus, present invention isdirected towards a photodiode and/or photodiode array having arelatively small p+ diffused active area, smaller than the area of thescintillator crystal mounted on the front side of the photodiode, and areflective metal shield, exhibiting improved device characteristics,such as low capacitance, low dark current, improved signal-to-noiseratio, low resistivity starting material, and lower fabrication andmanufacturing costs.

In one embodiment, the present invention is directed towards aphotodiode and/or that is fabricated on a low cost, low resistivitysubstrate wafer, ranging from 1000-2000 ohm cm while still retainingcapacitance performance of the conventional 2 mm×2 mm photodiodedescribed above, which is manufactured on an expensive, high resistivitysilicon wafer, ranging from 4500-9000 ohm cm. In one embodiment, the lowresistivity substrate wafer is a n-type silicon wafer. In oneembodiment, the low resistivity substrate wafer is a p-type siliconwafer.

In one embodiment, the present invention is directed towards aphotodiode having a relatively wide distance between the p+ diffusedarea and the edge of the photodiode, thus reducing sawing damage andincreasing yield and throughput of the device.

The present invention is directed toward multiple embodiments. While thepresent invention is described in detail with respect to an individualphotodiode element, it should be understood to those of ordinary skillin the art that a plurality of such photodiode elements may beaggregated on a substrate to form a photodiode array. Further, while thephotodiode and photodiode array of present invention is described withrespect to p+ diffused active areas on an n-type silicon wafer, itshould be noted and understood by those of ordinary skill in the artthat the present invention can be designed and manufactured with reversepolarity, and more specifically, n+ diffused active areas on p-typesilicon substrate wafers. Thus, the present invention is not limited tothe polarity presented herein.

Various modifications to the preferred embodiment, disclosed herein,will be readily apparent to those of ordinary skill in the art and thedisclosure set forth herein may be applicable to other embodiments andapplications without departing from the spirit and scope of the presentinvention and the claims hereto appended. Reference will now be made indetail to specific embodiments of the invention. Language used in thisspecification should not be interpreted as a general disavowal of anyone specific embodiment or used to limit the claims beyond the meaningof the terms used therein.

FIGS. 2 a and 2 b are top view and cross-sectional views, respectively,of one embodiment of a photodiode of the present invention. Photodiode200 comprises shallow p+ diffused region 210, which, in one embodiment,has an area smaller than the area of mounted scintillator crystal 220,shown in FIG. 2 b and described in greater detail below. In oneembodiment, p+ diffused region 210 has a top surface area of 1 mm(length) or less×1 mm (width) or less. In one embodiment, the topsurface area of p+ diffused region 210 is 1 mm×1 mm.

Further, photodiode 200 can be fabricated on a low cost, low resistivitysilicon wafer substrate 205, on the order of 1000-2000 ohm cm, whilestill retaining the capacitance performance of the conventional 2 mm×2mm p+ diffused area photodiode described in FIGS. 1 a and 1 b above,which is processed on relatively expensive higher resistivity 4500-9000ohm cm silicon material.

Persons having ordinary skill in the art should appreciate that the p+diffused area 210 is smaller, more specifically 1 mm×1 mm (versus 2 mm×2mm of the prior art photodiodes shown in FIGS. 1 a and 1 b) and thesubstrate material 205 has a lower resistivity, more specifically,ranging from 1000-2000 ohm cm (versus 4500-9000 ohm cm of the prior artphotodiodes shown in FIGS. 1 a and 1 b), and thus, photodiode 200 of thepresent invention has lower dark current and lower capacitance,resulting in an improved signal-to-noise ratio. In order to retain theperformance characteristics of the photodiode, while still employing arelatively small p+ diffused active area and a low resistivity startingmaterial, the photodiode further comprises a reflective metal shield225, described in further detail below.

Additionally, because of the relatively small p+ diffused area 210 ofthe photodiode of the present invention, there is a wider distancebetween the p+ diffused area and the edge of the photodiode chip, thanin the conventional photodiode described in FIGS. 1 a and 1 b, reducingsawing damage and increasing yield or throughput of the photodiode. Morespecifically and referring back to FIG. 1 a, in a typical photodiodechip, distance 129 between the p+ diffused area 110 and the edge 130 ofthe photodiode 100 ranges from 0.1 mm to 0.125 mm. Referring back toFIG. 2 a, however, in the photodiode chip 200 of the present invention,distance 229 between the p+ diffused area 210 and the edge 230 of thephotodiode 200 ranges from 0.3 mm to 0.5 mm, and is thus wider, allowingfor more sawing room.

Referring to FIGS. 2 a and 2 b, in one embodiment, photodiode 200further comprises deep diffused p+ region 212, which is a heavily dopedp+ region that is employed to reduce the resistance of the long neck 216of the device, as shown in FIG. 2 a.

In one embodiment, photodiode 200 further comprises anode metal layers(pads) 214 a and 214 b for forming contacts and for forming wire bonds,respectively. In one embodiment, metal layers 214 a and 214 b arefabricated from a high reflectance material, such as aluminum, having athickness of approximately 1 μm.

In one embodiment, photodiode 200 further comprises layer 215 (shownonly in FIG. 2 b), which is, in one embodiment, a relatively thin layercomprised of silicon oxide/silicon nitride that acts as junctionpassivation and antireflection layer. In one embodiment, layer 215 is ananti-reflective layer comprised of SiO₂ having a thickness of 900 Å. Inanother embodiment, layer 215 is an anti-reflective layer comprised ofboth SiO₂, having a thickness of 150 Å and Si₃N₄, having a thickness of400 Å, for a total layer thickness of 550 Å.

Referring now to FIG. 2 b, photodiode 200 includes a mountedscintillator crystal 220, which, in one embodiment, has a top surfacearea that is greater than the area of the p+ diffused active area 210.In one embodiment, the top surface area of mounted scintillator crystal220 is on the order of 2 mm×2 mm. Additionally, photodiode 200 furthercomprises metal shield 225, which is formed on at least a portion of thetop surface of the photodiode chip 200 and employed as a mirror toreflect incident light back into the scintillator crystal 220. In oneembodiment, metal shield 225 is formed from aluminum. In one embodiment,the metal shield 225 has the same dimensions and surface area as thescintillator crystal.

Thus, in one embodiment, the photodiode of the present invention is aphotodiode and/or photodiode array having a relatively small p+ diffusedactive area, in which at least a portion of the front or top surface ofthe photodiode or photodiode array is covered with a metal shield,employed as a reflective surface to reflect incident light back into ascintillator crystal mounted on the front side of the photodiode. In oneembodiment, the relatively small p+ diffused area that is smaller thanthe area of a scintillator crystal that is mounted on the top surface ofthe photodiode.

In one embodiment, the reflective metal shield increases the performanceof the photodiode and compensates for the smaller p+ diffused activearea in terms of performance characteristics, and more specifically, lowcapacitance. FIG. 3 illustrates how a photodiode or photodiode arraythat employs a reflective metal shield in conjunction with a p+ diffusedactive area that is smaller than the area of the mounted scintillatorcrystal can still exhibit the same or better performance characteristicsas a the conventional photodiode with a larger p+ diffused active areathat is equal to the area of the mounted scintillator crystal.

Referring now to FIG. 3, light reflected from the metal shieldundergoing multiple reflections from within the scintillator crystal 320is depicted, where light 305 travels at least one path 307. Light notinitially collected by p+ shallow diffused region 315 is reflected frommetal shield 325. It then creates incident light path 309 back into thescintillator crystal 320. The light undergoes multiple reflectionswithin the scintillator crystal and finally arrives at the 1 mm×1 mmdiffused area. Thereafter, it is collected by the 1 mm×1 mm p+ shallowdiffused region/active area 315.

An exemplary manufacturing process of one embodiment of the photodiodeof the present invention will now be described in greater detail.Persons of ordinary skill in the art should note that although oneexemplary manufacturing process is described herein, variousmodifications may be made without departing from the scope and spirit ofthe invention. Modifications or alterations to the manufacturing steps,their corresponding details, and any order presented may be readilyapparent to those of ordinary skill in the art. Thus, the presentinvention contemplates many possibilities for manufacturing thephotodiode of the present invention and is not limited to the examplesprovided herein.

FIGS. 4 a through 4 f depict cross-sectional views of exemplarymanufacturing steps of the photodiode of the present invention.Referring now to FIG. 4 a, the starting material is a device wafer 405,which, in one embodiment, is silicon, n-type wafer, having a resistivityranging from 1000-2000 ohm cm. In one embodiment, the n-type siliconsubstrate wafer has a thickness ranging from 300-400 μm. While it ispreferred that the wafer substrate be comprised of silicon, one ofordinary skill in the art would appreciate that any suitablesemiconductor material, which can be processed in accordance with themanufacturing steps of the present invention, may be used. In addition,device wafer 405 may optionally be polished on both the front side 405 aand back side 405 b to allow for greater conformity to parameters,surface flatness, and specification thickness. It should be understoodby those of ordinary skill in the art, however, that the abovespecifications are not binding and that the type of material andresistivity can easily be changed to suit the design, fabrication, andfunctional requirements of the present invention.

Still referring to FIG. 4 a, in step 441, device wafer 405 is subjectedto a standard mask oxidation process that grows a mask oxide layer 410on both the front side 405 a and back side 405 b of the device wafer405. In one embodiment, the oxidation mask is comprised of SiO₂ and/orSi₃N₄ and thermal oxidation is employed to achieve mask oxidation. Inone embodiment, the oxide layer 410 comprises silicon oxide (SiO₂)having a thickness of approximately 10,000 Å. Standard mask oxidation iswell known to those of ordinary skill in the art and will not bedescribed in further detail herein.

As shown in FIG. 4 b, in step 442 a, after the standard mask oxidationis complete, the device wafer 405 is subjected to n+ photolithography onthe front side 405 a and back side 405 b of device wafer 405. Afterselecting a suitable material and creating a suitable photoresistpattern, a thin photoresist layer is applied to the front side 405 a andback side 405 b of device wafer 405 to etch the pattern on the surfacesof the wafer. Generally, the photoresist layer is a photosensitivepolymeric material for photolithography and photoengraving that can forma patterned coating on a surface. In one embodiment, the photoresistlayer is applied via a spin coating technique. Spin coating is wellknown to those of ordinary skill in the art and will not be described indetail herein.

In step 442 b, the photoresist-coated device wafer 405 is aligned withan n+ mask. N+ masking techniques are employed to protect portions ofdevice wafer 405. Generally, photographic masks are high precisionplates containing microscopic images of preferred pattern or electroniccircuits. They are typically fabricated from flat pieces of quartz orglass with a layer of chrome on one side. The mask geometry is etched inthe chrome layer. In one embodiment, the n+ mask comprises a pluralityof diffusion windows with appropriate geometrical and dimensionalspecifications. The n+ mask allows selective irradiation of thephotoresist on the device wafer. Regions that are exposed to radiationare removed while those that are protected from diffusion remainshielded by the n+ mask.

An intense light, such as UV light, is projected through the mask,exposing portions of the photoresist layer in the pattern of the n+mask. The exposed and remaining photoresist is then subjected to asuitable chemical or plasma etching process to reveal the patterntransfer from the mask to the photoresist layer.

In one embodiment, the pattern of the photoresist layer and/or n+ maskdefines a plurality of regions 415, on the front side and back side ofdevice wafer 405, ready for n+ diffusion.

In step 442 c, device wafer 405 is subjected to oxide etching on boththe front side 405 a and back side 405 b to remove the silicon dioxidelayer deposited in step 441.

In step 442 d, the front side 405 a and back side 405 b of device wafer405 are subjected to n+ diffusion followed by drive-in oxidation afterthe previous n+ masking and etching steps. Generally, diffusionfacilitates propagation of a diffusing material through a host material.An appropriate amount of dopant atoms, such as phosphorous, is depositedonto the substrate wafer and fills the gaps left by the removedphotoresist layer. Then, the wafer 405 is subjected to a drive-inoxidation process that is used to redistribute the dopant atoms anddeposit them deeper into the wafer. In one embodiment, this processfills the plurality of regions 415, via deep diffusion, with n+ dopant.

In step 442 e, exposed silicon surfaces, such as the remaining portionsof front side 405 a and back side 405 b of device wafer 405, are coveredwith oxide layer 450. In one embodiment, oxide layer 450 has a thicknessof approximately 3,000 Å.

Referring now to FIG. 4 c, in step 443, front side 405 a of device wafer405 is subjected to a p+ lithography process, creating deep p+ diffusedregion 413. In one embodiment, the p+ dopant is boron. As with anyconventional lithography process, p+ lithography comprises at least thefollowing tasks, but not limited to such tasks: substrate preparation;photoresist application; soft baking; mask alignment; exposure;development; hard backing; and etching. In addition, various otherchemical treatments may be performed.

The p+ masking and diffusion process is similar to that delineated withrespect to the n+ masking process described earlier and will not berepeated in detail herein. The p+ masking process further comprisesdeposition and deep drive-in oxidation, allowing for predefined and/orpredetermined thermal budget in accord with the principles of thepresent invention. The exposed silicon surface of region 413 issubsequently oxidized.

In step 444, as shown in FIG. 4 d, the device wafer 405 is subjected toactive area photolithography and oxide etching, using an active areamask, to fabricate active area 414 on the front side 405 a.

Thereafter, an anti-reflective (AR) layer 417 is grown on the front sideof the device via thermal oxidation and/or CVD chemical deposition, suchas the thermal growth of a layer comprised of SiO₂, having a thicknessof 150 Å followed by the LPCVD deposition of Si₃N₄, having a thicknessof 400 Å, for a total layer thickness of 550 Å.

Persons of ordinary skill in the art would appreciate that variousanti-reflective coating designs, such as 1-layer, 2-layer, 3-layer, and4+-layer may be employed. By way of example, and by no means limiting,the anti-reflective coating design adopted herein utilizes thin filmmaterials, such as oxides, sulfides, fluorides, nitrides, selenides, andmetals, among others. In one embodiment of the present invention, theanti-reflective layer comprises silicon oxide/silicon nitride.

This is followed by boron implant and annealing, through the AR layer,to form a shallow p+ active area 416. The active area specifications,among other parameters, comprise significant performance characteristicsof the photodiode. In one embodiment, active area 414 is 1 mm×1 mm onthe top surface or front side 405 a of the photodiode.

Thereafter, in step 445, shown in FIG. 4 e, a contact etch mask is usedto etch a contact window 418 into the front side 405 a of wafer device405. The contact window 418 is formed using standard semiconductortechnology photolithography techniques.

Contact lithography, well known to those of ordinary skill in the art,involves printing an image or pattern via illumination of a photomask indirect contact with a substrate coated with an imaging photoresistlayer. Typically, a contact window is an aperture defined in a surfacepassivation layer through which device metallization develops contactwith circuit elements. In one embodiment, the contact window mask is adark field mask, which is used to remove the silicon oxide/siliconnitride layer in the region requiring contacts. While contact masks haveconventionally been fairly large (of the order of 100 mm or higher), itis possible that alignment tolerances may necessitate smaller mask sizesto allow stepping between exposures. As in nano-imprint lithography, themask needs to have roughly the same feature size as the desired image.

In one embodiment of the photodiode of the present invention, a contactwindow mask is first applied, followed by etching contact windowoxide/nitride on region 418. The contact window oxide/nitride used inthe process is subsequently removed by either standard wet or standarddry etching techniques as are well known to those of ordinary skill inthe art.

Using the contact mask, the contact window 418 is opened through theprotective passivation (AR) layer 417 deposited on the surface of thedevice wafer. In one embodiment, oxide and nitride and thus, contactwindow, etching is achieved via a chemical etching process, wherein thewafer is immersed in buffered oxide etch (BOE), a HF acid-based solutionfor intervals sufficient to remove the layers exposed by the contactwindow mask.

Referring now to FIG. 4 f, in step 446, the device wafer 405 issubjected to a metal deposition process to etch metal on the front side405 a and back side 405 b of device wafer 405 for creating electricalconnections. In one embodiment, the metal deposition process is alsoused to form a metal shield 420 on front side 405 a of device wafer 405,which is employed as a mirror to reflect incident light back into ascintillator crystal (not shown) that is mounted on the front sidephotodiode. The metal shield has been described in detail with respectto FIGS. 2 a, 2 b, and 3 and such details will not be repeated herein.

The most common metals include aluminum, nickel, chromium, gold,germanium, copper, silver, titanium, tungsten, platinum, and tantalum.Selected metal alloys may also be used. Metallization is oftenaccomplished with a vacuum deposition technique. The most commondeposition process include filament evaporation, electron-beamevaporation, flash evaporation, induction evaporation, and sputtering,followed by metal masking and etching.

In one embodiment, a metal mask is employed to achieve a patternmetallization. Aluminum etching is performed on the front side, to formanode contacts 419 and metal shield 420, using a variety of methods,including, but not limited to abrasive etching, dry etching,electroetching, laser etching, photo etching, reactive ion etching(RIE), sputter etching, and vapor phase etching. Similarly, chrome/goldmetallization is performed on the back side of the photodiode, thusforming the cathode 421.

The above examples are merely illustrative of the structure andmanufacturing steps of the photodiode array of the present invention.Although only a few embodiments of the present invention have beendescribed herein, it should be understood that the present inventionmight be embodied in many other specific forms without departing fromthe spirit or scope of the invention. Therefore, the present examplesand embodiments are to be considered as illustrative and notrestrictive, and the invention may be modified within the scope of theappended claims.

1. A photodiode comprising: a low resistivity substrate having at leasta front side and a back side, wherein said low resistivity is less than2000 ohm cm; a shallow p+ diffused region on said front side, whereinsaid shallow p+ region is smaller than a scintillator mounted on thefront side of the photodiode; at least one back side contact and atleast one front side contact, comprised of metal; and a reflective metalshield on at least a portion of said front side, wherein said metalshield reflects incident light back into said scintillator.
 2. Thephotodiode of claim 1 wherein said scintillator is a crystal and whereinsaid crystal is mounted on the front side of the photodiode.
 3. Thephotodiode of claim 1 wherein the shallow p+ diffused region is on theorder of 1 mm or less in length and 1 mm or less in width.
 4. Thephotodiode of claim 1 wherein the reflective metal shield comprisesaluminum.
 5. The photodiode of claim 1, wherein a distance between theshallow p+ diffused region and an edge of the photodiode ranges from 0.3mm to 0.5 mm.
 6. The photodiode of claim 1 further comprising ananti-reflective layer formed from SiO₂ having a thickness of 900 Å. 7.The photodiode of claim 1 further comprising an anti-reflective layerformed from SiO₂ having a thickness of 150 Å and Si₃N₄ having athickness of 400 Å, for a total layer thickness of 550 Å.
 8. Aphotodiode array comprising: a substrate having at least a front sideand a back side; a plurality of diode elements integrally formed in thesubstrate forming said array, wherein each diode element has a shallowp+ diffused region on said front side, wherein said shallow p+ region issmaller than a scintillator mounted on the front side of the photodiode;at least one front side contact, comprised of metal; and a reflectivemetal shield on at least a portion of said front side, wherein saidmetal shield reflects incident light back into the scintillator.
 9. Thephotodiode array of claim 8 wherein said scintillator comprises crystaland wherein said crystal is mounted on the front side of the photodiodeelement.
 10. The photodiode array of claim 8 wherein the shallow p+diffused region is approximately 1 mm or less in length.
 11. Thephotodiode of claim 1 wherein said low resistivity is greater than 1000ohm cm.
 12. The photodiode of claim 1 wherein said back side contact isa cathode contact.
 13. The photodiode of claim 1 wherein said front sidecontact is an anode contact.
 14. The photodiode of claim 1 wherein saidscintillator comprises a crystal, wherein said crystal has a top surfacearea of 2 mm×2 mm.
 15. The photodiode array of claim 8 wherein thesubstrate is a low resistivity substrate and wherein said lowresistivity is in the range of 1000 ohm cm to 2000 ohm cm.
 16. Thephotodiode array of claim 8 further comprising at least one back sidecathode contact.
 17. The photodiode array of claim 15 wherein said atleast one front side contact is an anode contact.
 18. The photodiodearray of claim 9 wherein said scintillator crystal has a top surfacearea of 2 mm×2 mm.
 19. The photodiode array of claim 10 wherein theshallow p+ diffused region is approximately 1 mm or less in width.